1. Field of the Invention
The present invention relates to dimmer control and in particular, to a dimmer control system and apparatus for controlling the illumination level of a fluorescent lamp.
2. Description of the Related Art
FIG. 1 shows a dimmer control circuit with a conventional silicon controlled rectifier (SCR) for an incandescent lamp RL. When switch K1 is turned on, (AC) power PS from the utility power is sent to a point A1 through resistor R2, switch K1, resistor R3 and variable resistor R1. Meanwhile, a capacitor C is charged during the positive half cycle of the AC power PS sine wave. While the voltage at point A1 in comparison with the voltage at point A reaches a turn-on voltage of the bi-directional diode Db3 and conditions at the two terminals of the SCR also meet certain turn on requirements, SCR is turned on and the incandescent lamp RL is illuminated. The incandescent lamp RL is also illuminated in the same manner during the negative half cycle of power PS sine wave.
Adjusting the resistance of the variable resistor R1 changes the charging speed for the capacitor C, the time point when the bi-directional diode Db3 is turned on, and, eventually, the turn-on angle at which SCR begins conduction. Since different turn-on angles respond to different average currents, average current through the incandescent lamp RL is controllable by adjusting the degree of the turn-on angle, such that the illumination level of the incandescent lamp is accordingly controlled or adjusted. FIG. 2 shows output waveforms of the SCR corresponding to different turn-on angles, wherein the black area in this figure indicates the conducting periods.
Unlike incandescent lamps, fluorescent lamps require an electronic ballast to regulate the flow of power. AC power cannot start the fluorescent lamp directly, and must be converted to direct current (DC) power. FIG. 3 shows a design for adjusting the illumination level of the fluorescent lamp by a SCR. In comparison with the circuit in FIG. 1, FIG. 3 has a bridge rectifier 4D and a capacitor CL, both added before the fluorescent lamp FL to convert the AC output of the SCR to DC output, which accordingly powers the fluorescent lamp FL.
When the loading of the SCR is capacitive, such as bridge rectifier 4D and capacitor CL shown in FIG. 3, the leakage of the SCR when turned off charges capacitor CL and causes reduction of the voltage across capacitor C1. Thus, the charge current of the capacitor C1 is reduced, and a phase shift between the charge current and the input of AC power PS stops voltages across the capacitor C1 from reaching the turn-on voltage of the bi-directional diode Db3 to turn on the SCR.
When the turn-on angle is small, inability of bi-directional diode Db3 and the SCR to be turned on frequently occur, such that the fluorescent light cannot function due to shortage of energy. As a result, the fluorescent lamp FL will flicker from repeated restarting.
Another conventional dimmer control circuit for a fluorescent lamp uses an extra control circuit in an electronic dimmer ballast. During power up, the electronic dimmer ballast generates a dimming signal to adjust the illumination of the fluorescent lamp. This configuration, however, requires extra circuit design to pass the dimming signal generated by the control circuit. The circuit for the fluorescent lamp, is thus complicated.